T he National Institute of Stan-dards and Technology (NIST) inMarch released the sixth in a

series of NIST technical notes (TN) onpenetration of radio signals into largebuilding structures, including apart-ment complexes, hotels, office build-ings, sports stadiums and shoppingmalls. The reports are intended to giveemergency responders and systemdesigners a better understanding ofwhat to expect from the radio-propaga-tion environment in emergency

response situations. The goal of theproject is to create a large set of public-domain data describing theattenuation and variability of radio sig-nals within various building types inthe public-safety frequency bands.

The first, second and third NISTTNs — NIST TNs 1540, 1541 and1542 — described experiments relatedto radio propagation in a structurebefore, during and after implosion. Thenext two notes — NIST TN 1545 and1546 — focused exclusively on RF

TECHNOLOGYTECHNOLOGYPUBLIC SAFETY

Top to bottom: Horizon West Apartments inBoulder, Colo.; Colorado Convention Center in Denver; NIST building in BoulderRight: Republic Plaza building in Denver

Researchers measure 700 MHz radio signal penetration into large buildings to track the propagation environment in emergency response situations.By Kate A. Remley, Christopher L. Holloway, William F. Young

In-BuildingSignal Tests

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signal penetration into large buildings,with no implosion results.

In the most recent NIST TN 1552,additional measurements were carriedout in the 750 MHz frequency band.Because the FCC is in the process ofauctioning the D block spectrum withplans for public-safety shared use,NIST researchers studied radio signalpenetration in this frequency bandinto four different large buildingstructures: a convention center, ahigh-rise office building, an apartmentbuilding and a laboratory/office build-ing. Two different types of signalmeasurements included:

■ Radio mapping, where RF trans-mitters were carried throughout the

structures and signals were received atfixed sites outside the structures, and

■ Broadband measurements thatprovided the time-response character-istics of the radio environment.

These latter measurements are use-ful in understanding the level ofreflectivity or multipath in a givenstructure. This article overviews themeasurement methods, an example ofthe measurement data collected and abrief interpretation of the results.

Measurement MethodsA fundamental challenge to radio

communications into and out of largebuildings is the strong attenuation of

radio signals caused by losses andscattering in a building’s materialsand structure. A second issue is thelarge amount of signal variabilitybecause of multipath that occursthroughout the large structures. TheNIST measurements were conductedin four large building structures in aneffort to quantify radio-signal attenua-tion and variability in scenariosencountered by emergency responseorganizations.

The NIST measurements wereintended to simulate an incidentcommand scenario where a fixed-location incident command station islocated outside a structure while amobile unit is carried within thebuilding. The data presented in theNIST study can be used to describethe radio channel from outside abuilding to inside in point-to-pointcommunications situations. Withsome additional modeling work, theycan also be used to develop modelsof emergency response trunked orcellular systems. In the latter case,the data would form the last leg ofthe communications link.

The first type of measurements,referred to as radio mapping, provideda continuous recording of receivedsignal strength at a fixed location out-side the structure while a transmitterwas carried through the structure. Thetransmitter emitted an unmodulated,749 MHz radio signal from within thestructure. The goal with these meas-urements was to cover as muchground in the structure as possible toprovide a detailed description of thestatistical variation in received signalstrength from signals transmitted with-in a given structure. By covering theroutes that would be walked by emer-gency responders within a structure,public-safety officials and researcherscan understand how strong or weak asignal will be, in terms of the average,maximum and minimum received sig-nal levels. Equally important, theexpected variance in signals in the 750MHz band can be determined.

The second type of measurementtested the time-domain response of the channel at particular locationswithin the buildings by use of a

Diagram of the lower floors of the Republic Plaza building, showing the radio-mappingtransmitter locations by black numbers, as well as the synthetic pulse measurements,shown by blue numbers inside the squares.

synthetic-pulse measurement system.These measurements used a vector net-work analyzer with its output port teth-ered to the receive antenna by a fiber-optic cable to allow reconstruction ofthe time-domain response of the propa-gation channel. With this system, wecan effectively reconstruct a short-duration pulse in post processing. Theshort pulse enables the study of themultipath in a given environment. Fig-ures of merit such as the root-mean-square (RMS) delay spread may becalculated and used to quantify thetime required for multipath reflectionsto decay below a given threshold level.

Penetration into StructuresLarge public buildings were chosen

for the research because they areexpected to present the biggest chal-lenges to emergency response commu-nications. For the measurements atRepublic Plaza, a 57-story officebuilding in downtown Denver, threereceive sites were used that variedsubstantially in distance from thebuilding. Receive site one was locatedabout 11 yards (10 meters) from thebuilding; receive site two was about27.3 yards (25 meters) from the build-ing; and receive site three was locatedon the roof of a parking garage about235 yards (215 meters) away. Theselocations were intended to simulatethe locations of command vehicles inan emergency response scenario.

The graphs on Page 38 show twoexamples of the radio mapping

results for the Republic Plaza build-ing. These graphs show the relativereceived signal levels for two differ-ent receive-site orientations withrespect to the building. The line-of-sight reference signal is clearly seenby the distinct largest peaks in theplots. In the table above, the addition-al free space path loss for receive sitethree is evident in the approximately18 dB reduction in the referencevalue compared with values obtainedat receive sites one and two. Thetable also shows that the variation inthe received signal levels at all threereceive sites given by the standarddeviation is similar. Note that the dataare normalized by the referencevalue, which is a line-of-sight signal,so that we can study the propagationeffects due to the building. The medi-an and standard deviation results arefor the normalized data.

Synthetic pulse measurements werealso carried out at positions marked bynumbers in rectangular boxes in thediagram, with the vector network ana-lyzer (VNA) located at receive siteone. We calculated the RMS delayspread from the data. Twenty-onepositions within the building were

tested, and 18 of those positions hadsufficient dynamic range to provideuseful results.

The RMS delay spread was calcu-lated for several different frequencybands. The measurements were madein the stairwell between floors. TheRMS delay spread was lowest at thelanding of each floor, where a windowwas located. The delay spreadincreased moderately as the heightincreased from around 50 nanosec-onds on floor one to around 150nanoseconds on the highest floor. Butwhen the measurements were madedeeper within the building on floorsfive and 10 (positions nine, 10 and16), the RMS delay spread increasedsignificantly to between 300 and 450nanoseconds, depending on the loca-tion and the frequency band used tocalculate the RMS delay spread.

Results from the StructuresThe measurement data showed a

wide range of received signal levelsand a high variability for these. Forthe radio mapping experiments, themedian values for all four buildingscalculated for data normalized to adirect line-of-sight path ranged from

Aggregate Statistics for Radio Mapping Results M edian (dB) Std. Dev (dB) Ref. (dBm )

Republic Plaza Receive Site One -68.3 20.3 -29.3

Republic Plaza Receive Site Two -60.0 21.5 -32.7

Republic Plaza Receive Site Three -49.3 17.8 -49.8

Republic Plaza building receive site three (left) and receive site one (right). Normalized received signal power as the 749 MHz transmitter iscarried through the building.

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-25.1 dB to -98.5 dB, and the corre-sponding standard deviation valuesranged from 6.8 dB to 30.1 dB. Thesefactors can complicate radio systemdesign because engineers need todesign systems that will provide reli-able reception in weak-signal condi-tions, but also systems that will tracksignals that vary from weak to strong.

The synthetic pulse measurementsshow that the level of reflectivity —multipath — in the structures was neg-ligible to moderate. The RMS delayspread results ranged from 15 to 450nanoseconds at various test locationswithin the four buildings. The RMSdelay values for measurements madein large open floor plan buildings weretwo to five times that of measurementsin buildings with relatively narrowcorridors. Many radio technologiescan handle this level of multipath,although these numbers could changesignificantly if a trunked or cellular

system design were used, rather thanthe point-to-point systems described inthis research.

The measured results provide keyparameters that describe the wirelesspropagation environment in represen-tative responder environments. NISThopes that improved channel descrip-tions provided by these measurementswill be useful for assessing current andfuture wireless technology in emer-gency scenarios, for standards develop-ment and for qualifying wireless equip-ment in environments such as thosestudied in this research project. ■

Kate A. Remley joined the Electromagnet-

ics Division, National Institute of Standards

and Technology (NIST), as an electronics

engineer in 1999. She is currently the edi-

tor-in-chief of IEEE Microwave Magazine.

Christopher L. Holloway has been with

NIST since 2000, where he works on elec-

tromagnetic theory. He is also on the grad-

uate faculty at the University of Colorado

at Boulder.

William F. Young has worked at Sandia

National Laboratories in Albuquerque,

N.M., since 1998, where he is currently a

principal member of the technical staff.

E-mail comments to editor@

RRMediaGroup.com.

Editor’s Note: This work was funded by

the NIST Public Safety Communications

Research (PSCR) Laboratory, through the

NIST Office of Law Enforcement Stan-

dards (OLES).

More Information■ For the full NIST TN 1552 and other technical notes, visit: www.nist.gov/eeel/electromagnetics/rf_fields/wireless.cfm

■ For more figures and graphs, visit www.MCCmag.com.